1. Field of the Invention
The invention relates to green luminophor materials having high luminous efficiency and luminous persistence or afterglow adapted to television applications and more specifically to high-definition plasma screens.
Indeed, in this type of application, it is sought to obtain luminous persistence values or times of decay, to 10% of the initial intensity emitted, typically lower than about fifteen milliseconds in order to avoid the trailing effect in the images.
2. Description of the Prior Art
At present, there are plasma panels available whose emitted radiation excites the luminophors capable of emitting in the visible range of the spectrum. The luminophors contain a reactive gas (a mixture of xenon and neon) which, by electrical discharge, produces light radiation in the short ultraviolet range, between 140 and 170 nm, also called VUV radiation. This light radiation is especially adapted to certain groups of luminophors, especially to barium hexa-aluminates which are already known to have high luminous efficiency under UV excitation, towards 250 nm. The color of emission of the luminophor is governed by the nature of the activator dispersed in the matrix. The activator is, in particular, europium for blue and manganese for green.
In the case of manganese-doped hexa-aluminates with the general formula Ba.sub.z -Al.sub.12-x Mn.sub.x O.sub.19 with 0.8.ltoreq.z.ltoreq.1.3, and x is .ltoreq.0.06 the emission is at 517 nm for luminous excitation in the neighborhood of 220 nm. For example, the commercial product Kx502 produced by Kyoko has a decay time of the order of 18 to 20 ms for a quantum yield in the neighborhood of 0.9. Analyses have shown that its manganese content x is in the region of 0.05.
This type of commercially available product has a decay time that is excessively long for the applications in view, where decay times of less than 15 ms and even 10 ms are sought.
Furthermore, it is known that to obtain a reduction of the decay time, the manganese content x can be increased.
Luminophors of this kind are manufactured according to a standard method that uses the heating of a mixture of oxides and salts containing the elements that constitute the luminophor. This heating is done in an oven, at a temperature such that the formation of the desired crystalline phase of the luminophor is obtained. In the specific case of barium hexa-aluminates, this is the spinel structure of .beta. alumina.
Nevertheless, tests carried out in non-sealed ovens under nitrogen atmosphere or under nitrogen and hydrogen reductive atmosphere show that increasing the manganese concentration fosters the creation of Mn.sup.3+ or Mn.sup.4+ ions to the detriment of Mn.sup.2+ ions. This phenomenon tends to reduce the luminous efficiency of the luminophor, inasmuch as the efficient ions in terms of luminous efficiency are the divalent Mn.sup.2+ ions.